1. Field
Stent like structures for insertion into body vessels, and the treatment of valve insufficiency.
2. Background
Generally speaking, oxygenated blood travels from the lungs to the left atrium by way of the pulmonary veins. The veins from the systemic circuit, the venae cavae and coronary sinus carry blood deficient in oxygen into the right atrium. The right ventricle takes blood received from the right atrium and sends it to the lungs, while the left ventricle takes blood received from the left atrium and sends it to the aorta.
The atrioventricular valves between respective ones of the atria and ventricles play important roles in the transport of blood through the body. The atrioventricular valves open during diastole, when the heart muscle relaxes, to allow blood to flow from the atria into the ventricles. The atrioventricular valves close during systole, when the heart muscle contracts, preventing the back flow of blood into the atria and allowing blood from the ventricles to be efficiently pumped into the lungs via the pulmonary tract and to the rest of the body via the aorta.
The mitral valve is the atrioventricular valve that controls blood flow from the left atrium into the left ventricle. The mitral valve is a bicuspid valve, describing the two cusps or leaflets that open and close the valve. The cusps or leaflets are attached to a muscular and fibrous ring around the orifice (mitral valve annulus) and their apices hang down into the left ventricle. When the ventricle fills with blood and begins to contract, the valve cusps or leaflets flow into position in the atrioventricular opening and are forced shut (coaptate) by the increasing pressure. To prevent the valve cusps or leaflets from turning into the left atrium and regurgitating blood, tendinous cords, the chordae tendineae, are attached to the free margins and ventricular surfaces of the cusps or leaflets. At the other ends, these cords attached to one of a respective pair of papillary muscles projecting from the ventricular wall. By contracting, these muscles maintain the integrity of the valve during ventricular contraction or systole.
When the two cusps or leaflets of the mitral valve do not completely close, there is backflow, or regurgitation of blood. The backflow increases the pressure in the left atrium which leads to pulmonary hypertension and dilation of the heart which are common symptoms of congestive heart failure. A heart then has to work harder pumping blood for the body which can lead to heart damage. Incomplete closing of the mitral valve cusps or leaflets is common, occurring generally in about seven percent of the population. Conditions contributing to incomplete closure of the mitral valve cusps or leaflets include genetic defects, infections, coronary artery disease, myocardial infarction, or congestive heart failure. These conditions contribute to mitral valve regurgitation resulting from enlargement of the mitral valve annulus and/or movement of the papillary muscles away from the valve as a result of ventricular enlargement. When the annulus enlarges, the cusps or leaflets of the valve are no longer able to close (coaptate), because the distance between the two cusps or leaflets has increased too much for the cusps or leaflets to touch each other and thus close off blood flow to the left atrium during, for example, systole. Mitral valve regurgitation can also result as a secondary etiology due to the remodeling of a distorted left ventricle in ischemic heart disease. It is known that as the ventricle is remodeled, the papillary muscles can be displaced away from their natural position. This displacement alters the natural tethering of the cusps or leaflets and restricts the ability of the cusps or leaflets to close properly at the level of the annulus.
In general, most cases of mitral valve regurgitation are mild and the symptoms may be controlled with drugs. In more serious cases, the mitral valve can be repaired through a procedure known as annuloplasty, a surgical procedure in which a synthetic ring is placed around the valve annulus. Annuloplasty encourages aptation of the mitral valve cusps or leaflets by shrinking the size of the valve opening. In other instances, a faulty mitral valve must be surgically replaced with a new valve. These surgical repairs require the opening of the chest by sternotomy or at best through small incisions in the chest wall, heart lung bypass and stopping the heart beat. In general, annuloplasty is an extremely invasive procedure, and, as such, a less invasive treatment for annular dilation is desirable.